Tissue culture of rabbit uterine endometrial and myometrial cells have been utilized to investigate the cellular origin of utero-placental coagulation factor XIII. An SV-40 transformation of these endometrial cells has been successful. Laser light scattering is being utilized to study rigidity of noncrosslinked and crosslinked clots of lamprey fibrin as well as the plasmin degradation of crosslinked bovine fibrin clots. Salmon plasmin of very high activity is being isolated and characterized.